Rival explanations for this year's surge in Treasury yields have circulated for months, but last week's brutal stock selloff debunked one of them, according to a senior economist.

The 10-year yield has eased in recent weeks as investors seek shelter in Treasuries. Before the latest dip, however, it climbed more than 100 basis points—equivalent to a full percentage point—soared above 5% and at one point hit its highest level in 24 years. Because the 10-year Treasury yield anchors borrowing costs across the economy, from mortgages to corporate credit, its swings matter well beyond trading floors.

The run began after the Iran war started in late February, sending oil prices higher and raising expectations that the Federal Reserve would hike rates to fight suddenly resurgent inflation.

At the same time, hyperscalers—the largest cloud computing companies—have been flooding the bond market with fresh debt to finance massive capital spending and the AI infrastructure boom, which has kept the U.S. economy running hot and inflation sticky. That supply has competed for investors' dollars as the Treasury Department taps the bond market to finance the federal government's $2 trillion budget deficit—and to pay interest on the $40 trillion in U.S. debt.

Some on Wall Street have downplayed the spike in yields, arguing that the deluge of AI debt has crowded out U.S. bonds and forced the Treasury to offer investors higher returns at auctions.

Robin Brooks, a senior fellow at the Brookings Institution, a Washington-based policy research organization, does not buy that explanation. In a Substack post on Saturday, he said the past week's market selloff represented a "natural experiment" that puts the competing arguments to the test.

"On one end of the spectrum are people like me who think this is a rising fiscal risk premium as our public debt grows out of control," he wrote. "On the other end are folks who say it's hyperscaler debt issuance that's pushing up yields, so there's no sign fiscal policy's in trouble."

Brooks said corporate borrowing is nowhere near where it was during the IT bubble of the early 2000s, while public borrowing is historically high and more likely driving up yields.

He pointed to the bond market's reaction to the latest monthly jobs report, which showed payrolls grew by more than double the consensus estimate.

Compared with previous upside job surprises, the two-year yield rose more than what hot economic growth could explain, undercutting the idea that U.S. growth depends on AI as well as the argument that AI is responsible for higher yields.

Brooks also cited the market-implied yield for a 10-year security 10 years from now, which gauges the premium investors demand for the risk of holding Treasuries long term. Such risks include the trajectory of U.S. debt and deficits. If the AI bubble were pushing up long-term yields, this "10y10y forward yield" would have fallen alongside the 10-year and two-year yields.

"It didn't and that in my mind further cements the case for the fiscal risk premium," he said. "The midterm elections and the AI sell-off that's ensued have given us the perfect natural experiment to disentangle what's going on with long-term yields. They're up because of mounting fiscal stress, not because of AI."

That forward measure offers readers a standing gauge of how heavily investors are pricing the long-term trajectory of U.S. debt as new fiscal data and AI-related bond issuance arrive.

Warnings about U.S. debt have been piling up recently. Last month, the Treasury Department's final tally on fiscal year 2026, which ended September 30, showed the federal budget deficit widened to $1.99 trillion, 12% worse than the $1.78 trillion gap in fiscal 2025.

That came as revenue increased 3% to $5.4 trillion, lagging behind a 6% bump in spending to $7.4 trillion. Social Security and Medicare were the biggest line items, but third place went to interest payments on the national debt, which topped defense spending and Medicaid.

Gross interest costs for fiscal 2026 came in at $1.36 trillion, according to Treasury data, with an average interest rate of 3.53%. That is more than double the rate in 2021, just before the Federal Reserve embarked on an aggressive tightening cycle to rein in inflation.

Net interest payments rose 8% to $1.05 trillion, outpacing increases in total spending, revenue and GDP—a pace of growth that exceeds real GDP growth of about % as well as nominal GDP growth of about 6%.

Fiscal 2026's net interest costs translate to about $2.9 billion a day, and the Peter G. Peterson Foundation estimated they represent 3.2% of GDP, breaking the previous record set in 1991.

"Higher interest begets more debt, and greater debt begets higher rates and higher interest. Eventually, the whole thing could come crashing down," Committee for a Responsible Federal Budget President Maya MacGuineas said in September.

"That's why Washington needs a fiscal intervention now, before it's too late. The President, the Treasury Secretary, and Congressional leadership need to understand the severity of this issue. There are no easy fixes, and we can't just wish this problem away."

This story was originally featured on Fortune.com.